111. Minimum Depth of Binary Tree

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

思路：与104. Maximum Depth of Binary Tree类似。

在左右子树都不为空的条件下，根节点的最小深度 = 左右子树的最小深度+1。如果左子树为空，则最小深度为右子树的深度+1。反之亦然。

Javascript代码:

var minDepth = function(root) {
    if(root == null) return 0;
    
    if(root.left == null) return minDepth(root.right)+1;
    if(root.right == null) return minDepth(root.left)+1;
    
    return Math.min(minDepth(root.left),minDepth(root.right)) + 1;
};